I do remember seeing that - and a few other vintage signs - near the Four-Level in February. (The signage in that area is fascinating, not only due to the US route history of the junction, which included Route 66, but also due to the lack of clarity whether all of the Harbor Freeway is part of Interstate 110 or not, which seems to have been improved somewhat in the last two years.)
I thought it switched designation just a bit south of the four-level, with a few erroneous I-110 shields north of that switch. I do not remember exactly where the switch is.
1963: http://www.cahighways.org/maps/1963routes.jpg
Although 60/70/99 still run concurrent (along with I-10, which is not noted on this map), the Pomona Freeway paralleling Valley Boulevard is shown as under construction.
that must be just before that freeway section opened, with US-60 getting rerouted onto it. I think the existence of the sign showing only US 70/99 on the San Berdoo implies that 60 was on the new freeway as a US highway in 1963, before getting switched to CA-60 in 1964.
This means that the 1963 photo has an erroneous sign: the one in the background should have a 60 shield. It's gotta be
somewhere, and if it's not 70/99...
but I do not believe there is an outline 60 on that sign. I can't verify that, but it looks to be just letters to me, referring to the older surface street.
I cannot tell if the foreground sign has a scraped-off 60. The sign is at most 4 years old in the picture, and likely newer*, and therefore wouldn't have had time to build up a residue of dust and grime on which the missing 60 shield would be visible.
(I say likely newer because that style of non-porcelain sign entered heavy use in 1963, with a single set of 1960 examples being the only older ones I know of - and, as far as I know, no black signs were made of that style.)
So the foreground sign is either (likely) a brand new 1963 sign with 10/70 the only shields it ever had - or it once had a scraped off 60 as well.
Here's what it looks like today - note that the first line is for Fourth Street, and the third line has greenout for "Pomona Fwy/Route 60":
oops, Fourth, not Soto. I misremembered. Silly me; I saw the sign three days ago. Okay, I was doing 90mph

I can't tell in the older photo if that third line had an outline US 60 shield, though I would be far from surprised if that was it.
I do not believe so. It would've been a
white US-60 shield, if the number was put on between August, 1962 and April, 1964 (which is likely when the freeway opened) - and Div Hwys used the latest spec (hit or miss

).
that implies the sign was patched twice in very rapid succession. First, to change the surface street (as seen in the '63 photo) to "Pomona Fwy [60]" (with a US shield), and then, around April, 1964, the replacement of the US shield with a state shield. If the US-60 shield were a separate piece of white porcelain (probable), as opposed to just being printed white, then the second replacement wasn't a patch. Instead, they unscrewed the US-60 shield and put up a CA-60.
On Monday, I saw no evidence of there ever being a US-60 shield on the patch, but, again, I was doing 90. Furthermore, if the patch was there only a few months, then it would've have developed a wear pattern. But, I do believe there was one, because US-60 had to be signed
somewhere.
(Unless of course they plain old forgot for a while, and put the Pomona Fwy SR-60 patch on only after April, 1964.)
So now the question remains - what is under the patch? The '63 map doesn't identify that road by name, and the sign does not look like it says "Valley Boulevard" (or any Boulevard at all) - so I'd be interested to know what it
does say.
I will have to email that photo to a friend of mine who could answer the "where's US-60" question.